Heat exchangers used to cool gases of all types are known in the art. By way of example only, and for purposes of illustration herein, many heat exchangers are adapted to cool exhaust gases (e.g., produced by internal combustion engines, gas turbines, or other exhaust producing processes or devices). In certain applications commonly referred to as exhaust gas recirculation (EGR), some portion of the exhaust gas produced by an engine is cooled and recirculated back to the intake manifold of the engine. The relatively inert exhaust gas is added to the fresh combustion air charge delivered to the intake manifold, and can serve to lower the combustion temperature within the engine, thereby reducing the rate of formation of NOx, an environmental pollutant. In order to achieve the foregoing in this exemplary application, it is typically necessary for the temperature of the recirculated exhaust to be substantially reduced prior to its re-entry into the engine, and one or more heat exchangers (EGR coolers or EGRC) are typically used to cool the recirculated exhaust.
Fouling of heat exchange surfaces is a known problem when heat exchangers are exposed to many types of gases. Fouling refers to the accumulation of matter on the heat exchange surfaces, which has a detrimental impact on heat exchanger performance. With reference again to the case of exhaust gas heat exchangers, for example, particulates that are entrained within the exhaust flow are deposited onto surfaces that are exposed to the exhaust. The accumulation of particulate on the surfaces adds an additional resistance to the transfer of heat energy from the exhaust gas to the cooling fluid of the heat exchanger, and increases the pressure drop through the heat exchanger by constricting the available flow area.
The impact of fouling is typically taken into account when sizing a heat exchanger for cooling by applying a fouling factor in the heat transfer calculation. The fouling factor decreases the effective overall heat transfer coefficient of the heat exchanger, in order to ensure that the heat exchanger will be appropriately sized for the required heat transfer capability when operating in a fouled state.
The fouling factor will vary with the specifics of each heat exchanger geometry, and will also vary with the conditions under which the heat exchanger is operated. Specifically, it is known that the fouling factor has an inverse relationship with the Reynolds number (Re) of the flow. As is known in the art, the Reynolds number relates the flow's inertial forces to the flow's viscous forces. The Reynolds number can be calculated by the equation:
      Re    =                  m        ·        D                    A        ·        μ              ,where m is the mass flow rate of the fluid, A is the cross-sectional area of the flow path, D is the hydraulic diameter of the flow path, and μ is the dynamic viscosity of the fluid.
In the design and sizing of heat exchangers in general, a common approach to improving the heat exchanger performance is to increase the surface area density, or the amount of extended surface per unit volume that is exposed to the heat exchange fluid. This approach will result in a decrease in the Reynolds number, since the hydraulic diameter of the channels will be reduced. In a heat exchanger exposed to exhaust gas, this decrease in Reynolds number will tend to increase the fouling factor, thereby reducing or even entirely eliminating the desired improvement in heat exchanger performance.
In light of the continuing need for heat exchangers operable in a fouled state, having a high surface area density, and/or having a reduced susceptibility to performance degradation due to fouling, improved heat exchangers continue to be welcome additions to the art.